1. Field of the Invention
The present invention relates to a liquid developer for use in developing electrostatic latent images in electrophotography, electrostatic printing etc.
2. Description of the Prior Art
The composition of the conventional liquid developer for electrophotography is such that the pigment particles and the additives such as resin etc which have such actions as to control the polarity of the pigment particles and give dispersibility in the carrier liquid or provide the fixability to a toner are dispersed in a highly insulating carrier liquid and in said carrier liquid the said pigment particles absorb the resins to form a toner. For instance, as a polarity controlling agent or a dispersing agent of the pigment particles such as carbon black, there are used vegetable oils such as linseed oil, soya bean oil and resins such as alkyl resin, polystyrene and acrylic resins, and these are mixed with the pigment to be kneaded and pulverized finely and then are dispersed in a carrier liquid e.g. a highly insulating organic solvent such as paraffin hydrocarbons to produce a liquid developer.
As a method for determining the polarity of the toner particle, in addition to a method in which the surface of the pigment particles is covered with a polarity controlling resin as described above, another method in which the charged condition is controlled by dissolving a surface active agent in a carrier liquid and causing it to be absorbed by the toner particle is known. The surface active agents used for such purpose are numerous and examples thereof are the metallic soaps such as cobalt naphthenate, manganese naphthenate and the alkyl benzene sulphonates such as calcium, sodium, barium and the like dodecylbenzene sulphonates and the phospholipids such as lecithin, cephalin, etc, but the method and effect of their use cannot be said to be always uniform, and because they are of low electric resistance when they are dissolved in a carrier liquid to use it to the degree at which although the electric resistance is lowered yet not to damage the electrostatic latent image which had been formed on the photosensitive body, the quantity of the surface active agent is subject to a strict limitation. Due to the above there is such a defect that sufficient quantity to give adequate electric charge to the toner particles cannot be added.
Also, those known materials which hitherto have been dissolved in a carrier liquid to impart a negative electric charge to the toner particles are not so numerous, and only lecithin, alkyl benzene calcium sulphonate, polyamide resin etc are known.